W3JDR wrote:
Fred,
You're exactly correct! That's why a piano tuner person can strike a tuning
fork and a piano key at the same time and hear the frequency difference as a
low beat note.

Hearing beats does not require non-linear or multiplicative mixing -
please see my separate reply to Fred.


As to Ian's comment...I don't think "adding" is the correct term either.

I was referring to what *audio* engineers call "mixing", which is
nothing else but simple adding or linear combining.

I agree with everything you say below...

"Multiplying" or "sampling" are more precise terms. A perfect balanced
unity-gain mixer actually uses one of the input signals to sample the other.
On the positive half cycle of the LO, one phase of the RF signal is sampled,
and on the other half cycle of the LO the opposite phase of the RF is
sampled. Mathematically, this is equivalent to multiplying the RF signal by
+1 or -1 on alternating half cycles of the LO.

...but the processes you describe are not what a straightforward audio
"mixing" desk does.

The device you describe above, an audio engineer would know as a
"modulator" or a "ring modulator". For example, the LO could be at a low
frequency, to get some kind of throbbing effect. Both RF and audio
engineers would agree, that is true modulation.

The difference is that RF engineers would also call that process
"mixing"... but audio engineers would not because, to in their
professional world, "mixing" means adding or linear combining.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek